In making tooth replacements, especially artificial tooth crowns and bridges, from plastic-veneered metal frames comprising dental alloys (noble metal alloys and non-noble metal alloys), the bond between the plastic and the metal is of particular significance. The art contains many proposals for solving the problem of providing a durable bond between plastic and metal to produce gap-free, durable plastic veneers.
German Patent 36 42 290 discloses a method for improving the adhesion of plastics to metals, in particular in the production of dental prostheses, by applying a silicon dioxide layer and then silanizing it. The silicon dioxide layer is applied by the application of a silica sol (mean primary particle size 5-150 nanometers) or a dispersion of a superfine silicic acid (mean primary particle size 5-50 namometers) to the metal surface and firing at a temperature of 100.degree. to 800.degree. C. The silica sols are aqueous colloidal dispersions of amorphous silicon dioxide, which is present in the form of single spherical particles that are not cross-linked to one another and have hydroxy groups on the surface. Pyrogenic silicic acids are particularly suitable for the dispersion of superfine silicic acids in water and/or alcohol as dispersing agents.
The disadvantages of the known methods include in particular (i) inadequate adhesion strength of the plastic to the many different known dental alloys, particularly under the conditions found in use in the mouth; and (ii) the excessive cost of the equipment for producing the bonds which are possible following known methods.